Yaw Control Systems for Tailsitting Biplane Aircraft

ABSTRACT

An aircraft includes an airframe with first and second wings having a fuselage extending therebetween. A propulsion assembly is coupled to the fuselage and includes a counter-rotating coaxial rotor system that is tiltable relative to the fuselage to generate a thrust vector. First and second yaw vanes extend aftwardly from the fuselage. A flight control system is configured to direct the thrust vector of the coaxial rotor system and control movements of the yaw vanes. In a VTOL orientation of the aircraft, differential operation of the yaw vanes and/or differential operations of first and second rotor assemblies of the coaxial rotor system provide yaw authority for the aircraft. In a biplane orientation of the aircraft, collective operation of the yaw vanes provides yaw authority for the aircraft.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to aircraft operable totransition between thrust-borne lift in a VTOL orientation andwing-borne lift in a biplane orientation and, in particular, to atailsitting biplane aircraft having a counter-rotating coaxial rotorsystem with thrust vectoring capabilities.

BACKGROUND

Fixed-wing aircraft, such as airplanes, are capable of flight usingwings that generate lift responsive to the forward airspeed of theaircraft, which is generated by thrust from one or more jet engines orpropellers. The wings generally have an airfoil cross section thatdeflects air downward as the aircraft moves forward, generating the liftforce to support the airplane in flight. Fixed-wing aircraft, however,typically require a runway that is hundreds or thousands of feet longfor takeoff and landing. Unlike fixed-wing aircraft, vertical takeoffand landing (VTOL) aircraft do not require runways. Instead, VTOLaircraft are capable of taking off, hovering and landing vertically. Oneexample of VTOL aircraft is a helicopter which is a rotorcraft havingone or more rotors that provide lift to the aircraft. The rotors notonly enable hovering and vertical takeoff and landing, but also enableforward, backward and lateral flight. These attributes make helicoptershighly versatile for use in congested, isolated or remote areas wherefixed-wing aircraft may be unable to takeoff and land. Helicopters,however, typically lack the forward airspeed and efficiency offixed-wing aircraft.

A tiltrotor aircraft is another example of a VTOL aircraft. Tiltrotoraircraft generate lift and propulsion using proprotors that aretypically coupled to nacelles mounted near the ends of a fixed wing. Thenacelles rotate relative to the fixed wing such that the proprotors havea generally horizontal plane of rotation for vertical takeoff, hoveringand landing and a generally vertical plane of rotation for forwardflight, wherein the fixed wing provides lift and the proprotors provideforward thrust. In this manner, tiltrotor aircraft combine the verticallift capability of a helicopter with the speed and range of fixed-wingaircraft. Tiltrotor aircraft, however, typically suffer from downwashinefficiencies during vertical takeoff and landing due to interferencecaused by the fixed wing. A further example of a VTOL aircraft is atiltwing aircraft that features a rotatable wing that is generallyhorizontal for forward flight and rotates to a generally verticalorientation for vertical takeoff and landing. Propellers are coupled tothe rotating wing to provide the required vertical thrust for takeoffand landing and the required forward thrust to generate lift from thewing during forward flight. The tiltwing design enables the slipstreamfrom the propellers to strike the wing on its smallest dimension, thusimproving vertical thrust efficiency as compared to tiltrotor aircraft.Tiltwing aircraft, however, are more difficult to control during hoveras the vertically tilted wing provides a large surface area forcrosswinds typically requiring tiltwing aircraft to have either cyclicrotor control or an additional thrust station to generate a moment.

SUMMARY

In one aspect, the present disclosure is directed to an aircraftoperable to transition between thrust-borne lift in a VTOL orientationand wing-borne lift in a biplane orientation. The aircraft includes anairframe with a first wing, a second wing and a fuselage that extendsbetween the first and second wings. A propulsion assembly is coupled tothe fuselage and includes a counter-rotating coaxial rotor system thatis tiltable relative to the fuselage to generate a thrust vector. Firstand second yaw vanes extend aftwardly from the fuselage. A flightcontrol system is configured to direct the thrust vector of the coaxialrotor system and control movements of the yaw vanes. In the VTOLorientation, differential operation of the yaw vanes provides yawauthority for the aircraft. In the biplane orientation, collectiveoperation of the yaw vanes provides yaw authority for the aircraft.

In certain embodiments, the propulsion assembly may include a motorassembly such that the coaxial rotor system and the motor assembly aretiltable relative to the fuselage to generate the thrust vector. In someembodiments, the coaxial rotor system may be configured to providethrust in line with a yaw axis of the aircraft, in the VTOL orientation,and in line with a roll axis of the aircraft, in the biplaneorientation. In certain embodiments, in the VTOL orientation, the firstwing is forward of the fuselage and the second wing is aft of thefuselage and, in the biplane orientation, the first wing is below thefuselage and the second wing is above the fuselage. In some embodiments,the first and second yaw vanes may be pivotably coupled to the fuselage.

In certain embodiments, in the VTOL orientation, the aircraft has a VTOLyaw axis that extends through the fuselage and the propulsion assemblyand, in the biplane orientation, the aircraft has a biplane yaw axisthat extends through the fuselage and the wings. In such embodiments, inthe VTOL orientation, the first and second yaw vanes may besymmetrically disposed relative to the VTOL yaw axis and, in the biplaneorientation, the first and second yaw vanes may be aftwardly disposedrelative to the biplane yaw axis. In some embodiments, in the VTOLorientation, the first yaw vane may be actuated in a first direction andthe second yaw vane may be actuated in a second direction that isopposite of the first direction to provide yaw authority for theaircraft. In certain embodiments, in the VTOL orientation, downwashgenerated by the propulsion assembly acting on the actuated first andsecond yaw vanes may create first and second yaw moments that urge theaircraft to rotate about the VTOL yaw axis. In some embodiments, in thebiplane orientation, the first and second yaw vanes may both be actuatedin a first direction to provide yaw authority for the aircraft. Incertain embodiments, in the biplane orientation, airflow generatedresponsive to the forward airspeed of the aircraft acting on theactuated first and second yaw vanes may create yaw moments that urgesthe aircraft to rotate about the biplane yaw axis.

In another aspect, the present disclosure is directed to an aircraftoperable to transition between thrust-borne lift in a VTOL orientationand wing-borne lift in a biplane orientation. The aircraft includes anairframe with a first wing, a second wing and a fuselage that extendsbetween the first and second wings. A propulsion assembly is coupled tothe fuselage and includes a counter-rotating coaxial rotor system havingfirst and second rotor assemblies. The coaxial rotor system is tiltablerelative to the fuselage to generate a thrust vector. First and secondyaw vanes extend aftwardly from the fuselage. A flight control system isconfigured to direct the thrust vector of the coaxial rotor system,control rotor speeds and collective pitches of the first and secondrotor assemblies and control movements of the yaw vanes. In the VTOLorientation, differential operation of the first and second rotorassemblies and differential operation of the yaw vanes provide yawauthority for the aircraft.

In some embodiments, in the VTOL orientation, the flight control systemmay be configured to provide yaw authority for the aircraft responsiveto actuating the first yaw vane in a first direction, actuating thesecond yaw vane in a second direction that is opposite of the firstdirection and differentially operating the first and second rotorassemblies by adjusting the rotor speeds. In certain embodiments, in theVTOL orientation, the flight control system may be configured to provideyaw authority for the aircraft responsive to actuating the first yawvane in a first direction, actuating the second yaw vane in a seconddirection that is opposite of the first direction and differentiallyoperating the first and second rotor assemblies by adjusting thecollective pitches. In some embodiments, in the VTOL orientation, theflight control system may be configured to provide yaw authority for theaircraft responsive to actuating the first yaw vane in a firstdirection, actuating the second yaw vane in a second direction that isopposite of the first direction and differentially operating the firstand second rotor assemblies by adjusting the rotor speeds and thecollective pitches.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1G are schematic illustrations of a tailsitting biplaneaircraft having a coaxial rotor system in accordance with embodiments ofthe present disclosure;

FIGS. 2A-2I are schematic illustrations of a tailsitting biplaneaircraft having a coaxial rotor system in a sequential flight operatingscenario in accordance with embodiments of the present disclosure;

FIGS. 3A-3F are schematic illustrations of a tailsitting biplaneaircraft having a coaxial rotor system in a sequential flight operatingscenario in accordance with embodiments of the present disclosure;

FIGS. 4A-4D are schematic illustrations of a tailsitting biplaneaircraft having a coaxial rotor system in various flight configurationsin accordance with embodiments of the present disclosure;

FIGS. 5A-5H are schematic illustrations of a tailsitting biplaneaircraft having a coaxial rotor system in various flight configurationsin accordance with embodiments of the present disclosure;

FIG. 6 depicts a propulsion assembly and gimbal assembly for atailsitting biplane aircraft having a coaxial rotor system in accordancewith embodiments of the present disclosure;

FIG. 7 depicts an aft door of a fuselage, in partial cutaway, for atailsitting biplane aircraft having a coaxial rotor system in accordancewith embodiments of the present disclosure;

FIGS. 8A-8C depict a tail assembly in various operating configurationsfor a tailsitting biplane aircraft having a coaxial rotor system inaccordance with embodiments of the present disclosure;

FIG. 9A is a systems diagram of one implementation of a tailsittingbiplane aircraft having a coaxial rotor system in accordance withembodiments of the present disclosure;

FIG. 9B is a block diagram of autonomous and remote control systems fora tailsitting biplane aircraft having a coaxial rotor system inaccordance with embodiments of the present disclosure;

FIGS. 10A-10D are schematic illustrations of a tailsitting biplaneaircraft having a coaxial rotor system in various roles in accordancewith embodiments of the present disclosure; and

FIGS. 11A-11D are schematic illustrations of various tailsitting biplaneaircraft having a coaxial rotor system in accordance with embodiments ofthe present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,not all features of an actual implementation may be described in thepresent disclosure. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would be a routine undertakingfor those of ordinary skill in the art having the benefit of thisdisclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicedescribed herein may be oriented in any desired direction. As usedherein, the term “coupled” may include direct or indirect coupling byany means, including moving and/or non-moving mechanical connections.

Referring to FIGS. 1A-1G in the drawings, a tailsitting biplane aircraftoperable to transition between thrust-borne lift in a VTOL orientationand wing-borne lift in a biplane orientation is schematicallyillustrated and generally designated as aircraft 10. FIGS. 1A, 1C and 1Edepict aircraft 10 in the VTOL orientation wherein the propulsionassembly provides thrust-borne lift enabling aircraft 10 to accomplishvertical takeoffs, hover and vertical landings. FIGS. 1B, 1D and 1Fdepict aircraft 10 in the biplane orientation wherein the propulsionassembly provides forward thrust with the forward airspeed of aircraft10 providing wing-borne lift enabling aircraft 10 to have a high speed,high efficiency and/or high endurance forward flight mode. Aircraft 10has a longitudinal axis 10 a that may be referred to as the roll axis, alateral axis 10 b that may be referred to as the pitch axis and avertical axis 10 c that may be referred to as the yaw axis, as best seenin FIGS. 1E and 1F. When longitudinal axis 10 a and lateral axis 10 bare both in a horizontal plane and normal to the local vertical in theearth's reference frame, aircraft 10 has a level flight attitude.

In the illustrated embodiment, aircraft 10 has an airframe 12 includingwings 14, 16 each having an airfoil cross-section that generates liftresponsive to the forward airspeed of aircraft 10, in the biplaneorientation. Wings 14, 16 may be formed as single members or may beformed from multiple wing sections. The outer skins of wings 14, 16 arepreferably formed from high strength and lightweight materials such asfiberglass, carbon, plastic, metal or other suitable material orcombination of materials. As best seen in FIG. 1B, wing 14 has adihedral configuration and wing 16 has an anhedral configuration. In theillustrated embodiment, wing 14 has a dihedral angle of about twelvedegrees and wing 16 has an anhedral angle of about twelve degrees. Inother embodiments, wing 14 may have a dihedral angle of between eightdegrees and sixteen degrees or another suitable dihedral angle.Likewise, wing 16 may have an anhedral angle between eight degrees andsixteen degrees or other suitable anhedral angle. The dihedral andanhedral configuration of wings 14, 16 provide enhanced ground stabilityof aircraft 10, while the dual wing design provides a compact footprinton the ground.

As best seen in FIG. 1E, wings 14, 16 have swept wing designs. In theillustrated embodiment, wings 14, 16 have a quarter chord sweep anglebetween fifteen degrees and thirty degrees such as a quarter chord sweepangle between twenty degrees and twenty-five degrees or a quarter chordsweep angle of about twenty-two degrees. In the illustrated embodiment,the leading edge sweep angle is greater than the quarter chord sweepangle and is about twenty-five degrees, the half chord sweep angle isless than the quarter chord sweep angle and is about nineteen degreesand the trailing edge sweep angle is less than the half chord sweepangle and is about twelve and half degrees. As illustrated, the sweepangle progressively decreases from the leading edge to the trailing edgeforming a tapered swept wing design. In other embodiments, the sweepangle may remain constant from the leading edge to the trailing edgeforming a simple swept wing design, the leading edge may have a sweepangle and the trailing edge may not have a sweep angle forming a deltaswept wing design or the leading edge may have a positive sweep angleand the trailing edge may have a negative sweep angle forming atrapezoidal swept wing design.

Airframe 12 also includes a fuselage 18 that extends generallyperpendicularly between wings 14, 16. Fuselage 18 is preferably formedfrom high strength and lightweight materials such as fiberglass, carbon,plastic, metal or other suitable material or combination of materials.As best seen in FIG. 1G, fuselage 18 has an aerodynamic shape tominimize drag during high speed forward flight. In addition, fuselage 18preferably has a length in the longitudinal direction configured tominimize interference drag between wings 14, 16. For example, thelongitudinal length of fuselage 18 may have a ratio to the wingspan ofwings 14, 16 of between 1 to 2 and 1 to 3 such as a ratio of about 1 to2.5. Fuselage 18 has an upper flange 18 a configured to receive wing 14and a lower flange 18 b configured to receive wing 16. In theillustrated embodiment, wing 14 is aft of fuselage 18 and wing 16 isforward of fuselage 18 in the VTOL orientation and wing 14 is abovefuselage 18 and wing 16 is below fuselage 18 in the biplane orientation.Wings 14, 16 may be attachable to and detachable from fuselage 18 andmay be standardized and/or interchangeable units and preferably linereplaceable units providing easy installation and removal from fuselage18. The use of line replaceable wings is beneficial in maintenancesituations if a fault is discovered with a wing. In this case, thefaulty wing can be decoupled from fuselage 18 by simple operations andanother wing can then be attached to fuselage 18. In other embodiments,wings 14, 16 may be permanently coupled to fuselage 18. In either case,the connections between wings 14, 16 and fuselage 18 are preferablystiff connections. In the illustrated embodiment, fuselage 18 includesan aft door 18 c that is pivotably coupled to main body 18 d of fuselage18. Aft door 18 c may be rotated relative to main body 18 d to allowaccess to the inside of fuselage 18, for cargo, passengers, crew or thelike. During flight operations, aft door 18 c is secured to main body 18d to prevent relative rotation therebetween. Operation of aft door 18 cbetween the open and closed positions may be manual or automated. Inother embodiments, an aft door for fuselage 18 may be formed from a pairof clamshell doors each of which is rotatable relative to main body 18 dto allow access to the inside of fuselage 18.

As best seen in FIGS. 1C and 1D, fuselage 18 houses a flight controlsystem 20 of aircraft 10. Flight control system 20 is preferably aredundant digital flight control system including multiple independentflight control computers. For example, the use of a triply redundantflight control system 20 improves the overall safety and reliability ofaircraft 10 in the event of a failure in flight control system 20.Flight control system 20 preferably includes non-transitory computerreadable storage media including a set of computer instructionsexecutable by one or more processors for controlling the operation ofaircraft 10. Flight control system 20 may be implemented on one or moregeneral-purpose computers, one or more special purpose computers orother machines with memory and processing capability. For example,flight control system 20 may include one or more memory storage modulesincluding, but not limited to, internal storage memory such as randomaccess memory, non-volatile memory such as read only memory, removablememory such as magnetic storage memory, optical storage, solid-statestorage memory or other suitable memory storage entity. Flight controlsystem 20 may be a microprocessor-based system operable to executeprogram code in the form of machine-executable instructions. Inaddition, flight control system 20 may be selectively connectable toother computer systems via a proprietary encrypted network, a publicencrypted network, the Internet or other suitable communication networkthat may include both wired and wireless connections.

Aircraft 10 includes a propulsion assembly 22 that is coupled tofuselage 18. Propulsion assemblies such as propulsion assembly 22 may beattachable to and detachable from fuselage 18 and may be standardizedand/or interchangeable units and preferably line replaceable unitsproviding easy installation and removal from fuselage 18. The use ofline replaceable propulsion assemblies is beneficial in maintenancesituations if a fault is discovered with a propulsion assembly. In thiscase, the faulty propulsion assembly can be decoupled from fuselage 18by simple operations and another propulsion assembly can then beattached to fuselage 18. In the illustrated embodiment, propulsionassembly 22 includes a counter-rotating coaxial rotor system 24 that hasfirst and second rotors assemblies 24 a, 24 b that share a common axisof rotation and counter-rotate relative to one another. First and secondrotors assemblies 24 a, 24 b may be referred to as upper rotor assembly24 a and lower rotor assembly 24 b in the VTOL orientation of aircraft10 and as forward rotor assembly 24 a and aft rotor assembly 24 b in thebiplane orientation of aircraft 10.

In the VTOL orientation of aircraft 10, when coaxial rotor system 24 isnot thrust vectoring, upper rotor assembly 24 a and lower rotor assembly24 b rotate about yaw or vertical axis 10 c, as best seen in FIG. 1E,providing thrust in line with the center of gravity of aircraft 10 andin line with yaw axis 10 c. In the biplane orientation of aircraft 10,when coaxial rotor system 24 is not thrust vectoring, forward rotorassembly 24 a and aft rotor assembly 24 b rotate about roll orlongitudinal axis 10 a, as best seen in FIG. 1F, providing thrust inline with the center of gravity of aircraft 10 and in line with rollaxis 10 a. In the illustrated embodiment, rotors assemblies 24 a, 24 bare rigid rotor systems and/or hingeless rotor systems. In otherembodiments, rotors assemblies 24 a, 24 b could have alternate rotorsystem designs such as fully or partially articulated rotor systems. Asillustrated in FIG. 1E, in the VTOL orientation, yaw axis 10 c extendsthrough fuselage 18 and propulsion assembly 22 and may be referred to asVTOL yaw axis 10 c. Likewise, roll axis 10 a extends through fuselage 18and wings 14, 16 and may be referred to as VTOL roll axis 10 a. Asillustrated in FIG. 1F, in the biplane orientation, yaw axis 10 cextends through fuselage 18 and wings 14, 16 and may be referred to asbiplane yaw axis 10 c. Likewise, roll axis 10 a extends through fuselage18 and propulsion assembly 22 and may be referred to as biplane rollaxis 10 a.

As best seen in FIG. 1B, forward rotor assembly 24 a has four rotorblades and aft rotor assembly 24 b has four rotor blades with forwardrotor assembly 24 a rotating counterclockwise, as indicated by arrow 24c, and with aft rotor assembly 24 b rotating clockwise, as indicated byarrow 24 d, when aircraft 10 is viewed from the front. In theillustrated embodiment, each rotor blade has a root to tip twist betweenforty degrees and fifty degrees such as a root to tip twist betweenforty-five degrees and forty-eight degrees or a root to tip twist ofabout forty-seven degrees. Forward rotor assembly 24 a and aft rotorassembly 24 b may have the same or similar diameters or may havedifferent diameters such as forward rotor assembly 24 a having a largerdiameter than aft rotor assembly 24 b. In the illustrated embodiment,the rotor disk of coaxial rotor system 24 has a ratio to the wingspan ofwings 14, 16 of between 1 to 1 and 1 to 3 such as a ratio of about 1 to2.5. In other embodiments, the rotor disk of coaxial rotor system 24could have a ratio to the wingspan of wings 14, 16 of greater than 1 to1 or less than 1 to 3.

In the illustrated embodiment, aircraft 10 is an electric verticaltakeoff and landing (eVTOL) aircraft having two electric motors depictedas a motor assembly 22 a, with each of the electric motors driving oneof forward rotor assembly 24 a and aft rotor assembly 24 b. In otherembodiments, aircraft 10 may have a single electric motor driving bothforward rotor assemblies 24 a, 24 b or may have one or more internalcombustion engines driving rotor assemblies 24 a, 24 b via one or moresuitable transmissions. In the illustrated embodiment, forward rotorassembly 24 a and aft rotor assembly 24 b are independently controllablerotor assemblies configured for independent variable speed control andindependent collective pitch control with no cyclic pitch control. Inother embodiments, rotor assemblies 24 a, 24 b could operate at aconstant speed, could have fixed pitch rotor blades and/or could havecyclic pitch control. In the illustrated embodiment, propulsion assembly22 including coaxial rotor system 24 and motor assembly 22 a is tiltablerelative to fuselage 18 to provide omnidirectional thrust vectoringcapability to aircraft 10 with the counter rotation of rotor assemblies24 a, 24 b cancelling the gyroscopic moments. For example, propulsionassembly 22 may be gimbal mounted to fuselage 18 via propulsion assemblyhousing 22 b, which is part of airframe 12, such that coaxial rotorsystem 24 and motor assembly 22 a tilt about orthogonal pivot axes suchas a pitch pivot axis and a lateral pivot axis. In other embodiments,propulsion assembly 22 may be gimbal mounted to fuselage 18 such thatcoaxial rotor system 24 tilts about two non-orthogonal pivot axes orsuch that coaxial rotor system 24 tilts about only one pivot axis suchas the pitch pivot axis. In still other embodiments, coaxial rotorsystem 24 may be tiltable relative to fuselage 18 with motor assembly 22a being non-tiltable.

Aircraft 10 has a distributed array of control surfaces carried by tailassemblies 26 a, 26 b, 26 c, 26 d, which may collectively be referred toas tail assemblies 26. In the illustrated embodiment, tail assemblies 26a, 26 b are respectively coupled to wingtips 14 a, 14 b of wing 14 andtail assemblies 26 c, 26 d are respectively coupled to wingtips 16 a, 16b of wing 16 such that tail assemblies 26 a, 26 b, 26 c, 26 d arepositioned outboard of the rotor disk of coaxial rotor system 24. Tailassemblies 26 may be independently attachable to and detachable from thewingtips and may be standardized and/or interchangeable units andpreferably line replaceable units providing easy installation andremoval from the wingtips. The use of line replaceable tail assembliesis beneficial in maintenance situations if a fault is discovered withone of the tail assemblies. In this case, the faulty tail assembly canbe decoupled from the wingtip by simple operations and another tailassembly can then be attached to the wingtip. In other embodiments, tailassemblies 26 may be permanently coupled to wings 14, 16.

Tail assembly 26 a includes a pair of aerosurfaces depicted as avertical stabilizer 28 a and an elevon 30 a. Tail assembly 26 b includesa pair of aerosurfaces depicted as a vertical stabilizer 28 b and anelevon 30 b. Tail assembly 26 c includes a pair of aerosurfaces depictedas a vertical stabilizer 28 c and an elevon 30 c. Tail assembly 26 dincludes a pair of aerosurfaces depicted as a vertical stabilizer 28 dand an elevon 30 d. Vertical stabilizers 28 a, 28 b, 28 c, 28 d maycollectively be referred to as vertical stabilizers 28 and elevons 30 a,30 b, 30 c, 30 d may collectively be referred to as elevons 30. In theillustrated embodiment, vertical stabilizers 28 are fixed aerosurfaces.In other embodiments, vertical stabilizers 28 could operate as rudders.In the illustrated embodiment, elevons 30 are pivoting aerosurfaces thatare rotatable about respective elevon axes that may be generallyparallel with wings 14, 16 at the respective dihedral and anhedralangles. When operated collectively, elevons 30 serve as elevators tocontrol the pitch or angle of attack of aircraft 10, in the biplaneorientation. When operated differentially, elevons 30 serve as aileronsto control the roll or bank of aircraft 10, in the biplane orientation.

Aircraft 10 includes a plurality of electrical power sources depicted asbatteries 32 a, 32 b, 32 c, 32 d, which may collectively be referred toas batteries 32. In the illustrated embodiment, batteries 32 form adistributed power system in which each battery 32 a, 32 b, 32 c, 32 d islocated in a receiving pocket of one of the tail assemblies 26 a, 26 b,26 c, 26 d such that batteries 32 provide inertial relief to wings 14,16. Batteries 32 provide power to flight control system 20, propulsionassembly 22 and other power consumers via a power management systemincluding, for example, a centralized DC bus. Alternatively oradditionally, batteries may be housed within fuselage 18 and/or wings14, 16. In some embodiments, aircraft 10 may have a hybrid power systemthat includes one or more internal combustion engines and an electricgenerator. Preferably, the electric generator is used to chargebatteries 32. In other embodiments, the electric generator may providepower directly to the power management system and/or the power consumerssuch as propulsion assembly 22. In still other embodiments, aircraft 10may use fuel cells as the electrical power sources. The fuel cell may belocated in the receiving pockets of tail assemblies 26, in fuselage 18and/or in wings 14, 16.

Aircraft 10 includes a pair of yaw vanes 34 a, 34 b that are pivotablycoupled to an aft end of fuselage 18. Yaw vanes 34 a, 34 b may beoperated differentially to generate yaw moments when aircraft 10 is inthe VTOL orientation and may be operated collectively to generate yawmoments when aircraft 10 is in the biplane orientation. Aircraft 10 hasa plurality of landing gear assemblies 36 a, 36 b, 36 c, 36 d that maycollectively be referred to as landing gear assemblies 36. Landing gearassemblies 36 a, 36 b, 36 c, 36 d are positioned at the distal end ofrespective tail assemblies 26 a, 26 b, 26 c, 26 d. The landing gearassemblies 36 may be passively operated pneumatic landing struts oractively operated telescoping landing struts. In other embodiments,landing gear assemblies 36 may include wheels that enable aircraft 10 totaxi and perform other ground maneuvers. In such embodiments, landinggear assemblies 36 may provide a passive brake system or may includeactive brakes such as an electromechanical braking system or a manualbraking system to facilitate parking during ground operations.

Aircraft 10 may be a manned or unmanned aircraft. Flight control system20 may autonomously control some or all aspects of flight operations foraircraft 10. Flight control system 20 is also operable to communicatewith remote systems, such as a ground station via a wirelesscommunications protocol. The remote system may be operable to receiveflight data from and provide commands to flight control system 20 toenable remote flight control over some or all aspects of flightoperations for aircraft 10. The remote flight control and/or autonomousflight control may be augmented or supplanted by onboard pilot flightcontrol during manned missions. Regardless of the input, aircraft 10preferably utilizes a fly-by-wire system that transmits electronicsignals from flight control system 20 to the actuators of controlledsystems to control the flight dynamics of aircraft 10 includingcontrolling the movements of elevons 30, yaw vanes 34 a, 34 b andpropulsion assembly 22. Flight control system 20 communicates with thecontrolled systems via a fly-by-wire communications network withinairframe 12. In addition, flight control system 20 receives data from aplurality of sensors 40 such as one or more position sensors, attitudesensors, speed sensors, altitude sensors, heading sensors, environmentalsensors, fuel sensors, temperature sensors, location sensors and thelike to enhance flight control capabilities.

Referring additionally to FIGS. 2A-2I in the drawings, a sequentialflight-operating scenario of aircraft 10 will now be described. As bestseen in FIG. 2A, aircraft 10 is in a tailsitter position on a surfacesuch as the ground or the deck of an aircraft carrier. When aircraft 10is ready for a mission, flight control system 20 commences operationsproviding flight commands to the various systems of aircraft 10. Flightcontrol system 20 may be operating responsive to autonomous flightcontrol, remote flight control, onboard pilot flight control or acombination thereof. For example, it may be desirable to utilize onboardpilot or remote flight control during certain maneuvers such as takeoffsand landings but rely on autonomous flight control during hover, highspeed forward flight and transitions between wing-borne flight andthrust-borne flight.

As best seen in FIG. 2B, aircraft 10 has performed a vertical takeoffand is engaged in thrust-borne lift in the VTOL orientation of aircraft10. As illustrated, upper rotor assembly 24 a and lower rotor assembly24 b are counter-rotating in generally parallel horizontal planes. Aslongitudinal axis 10 a and lateral axis 10 b (denoted as the target) areboth in a horizontal plane H that is normal to the local vertical in theearth's reference frame, aircraft 10 has a level flight attitude. In theVTOL orientation, wing 16 is the forward wing and wing 14 is the aftwing. Flight control system 20 independently controls and operates upperrotor assembly 24 a and lower rotor assembly 24 b includingindependently controlling rotor speed and collective pitch. In addition,flight control system 20 controls the tilt of propulsion assembly 22relative to fuselage 18 to generate a thrust vector.

During hover, flight control system 20 may utilize speed control and/orcollective pitch control of upper rotor assembly 24 a and lower rotorassembly 24 b to cause aircraft 10 to climb, descend or maintain astable hover. Also during hover, flight control system 20 may utilizethrust vectoring of propulsion assembly 22 to provide translationauthority for aircraft 10. For example, as best seen in FIG. 4A,propulsion assembly 22 is tiltable forward and aftward relative tofuselage 18 to provide translation authority to aircraft 10 in thefore/aft direction, as indicated by arrow 42. When propulsion assembly22 is tilted aftward relative to fuselage 18, as indicated by dottedpropulsion assembly 22 c, propulsion assembly 22 generates a thrustvector having a vertical component 44 providing thrust-borne lift foraircraft 10 and an aftward component 46 that urges aircraft 10 totranslate in the aftward direction. When propulsion assembly 22 istilted forward relative to fuselage 18, as indicated by dottedpropulsion assembly 22 d, propulsion assembly 22 generates a thrustvector having vertical component 44 providing thrust-borne lift foraircraft 10 and a forward component 48 that urges aircraft 10 totranslate in the forward direction.

As another example, as best seen in FIG. 4B, propulsion assembly 22 istiltable to the right and to the left relative to fuselage 18 to providetranslation authority to aircraft 10 in the lateral direction, asindicated by arrow 50. When propulsion assembly 22 is tilted to theright relative to fuselage 18, as indicated by dotted propulsionassembly 22 e, propulsion assembly 22 generates a thrust vector having avertical component 52 providing thrust-borne lift for aircraft 10 and alateral component 54 that urges aircraft 10 to translate to the right.When propulsion assembly 22 is tilted left relative to fuselage 18, asindicated by dotted propulsion assembly 22 f, propulsion assembly 22generates a thrust vector having vertical component 52 providingthrust-borne lift for aircraft 10 and a lateral component 56 that urgesaircraft 10 to translate to the left.

In the illustrated embodiment, the thrust vectoring capability ofpropulsion assembly 22 is achieved by operating a gimbal assembly 60, asbest seen in FIG. 6. Gimbal assembly 60 includes an inner gimbal ring 62that is coupled to motor assembly 22 a that includes electric motors 22g, 22 h that respectively provide torque and rotational energy to upperrotor assembly 24 a and lower rotor assembly 24 b. Gimbal assembly 60also includes an outer gimbal ring 66 that is rotatably coupled topropulsion assembly housing 22 b and rotatably coupled to inner gimbalring 62. An inner gimbal ring actuator 70 is configured to tilt innergimbal ring 62 relative to outer gimbal ring 66 about pitch pivot axis72 via linkage 74, responsive to commands from flight control system 20.An outer gimbal ring actuator 76 is configured to tilt outer gimbal ring66 relative to propulsion assembly housing 22 b about lateral pivot axis78 via linkage 80, responsive to commands from flight control system 20.In this manner, propulsion assembly 22 including coaxial rotor system 24and motor assembly 22 a is tilted relative to fuselage 18 to generatethe thrust vector. Even though aircraft 10 has been depicted in FIG. 4Aand described in reference thereto as being configurable for fore/afttranslation and even though aircraft 10 has been depicted in FIG. 4B anddescribed in reference thereto as being configurable for lateraltranslation, it should be understood by those having ordinary skill inthe art that the orthogonal pivot axes of gimbal assembly 60 provide fortilting of propulsion assembly 22 in any radial direction relative tofuselage 18 such that propulsion assembly 22 has omnidirectional thrustvectoring capability and such that aircraft 10 has omnidirectionaltranslation capability in hover.

Continuing with the sequential flight-operating scenario, aircraft 10remains in the hover operation in FIG. 2B. During hover, flight controlsystem 20 may utilize differential rotor speed, differential collectivepitch and/or differential yaw vane positioning to provide yaw authorityfor aircraft 10. For example, to maintain a stable hover, differentialrotor speed may be used wherein the average rotor speed of upper rotorassembly 24 a and lower rotor assembly 24 b is held constant whileincreasing the rotor speed of one of upper rotor assembly 24 a and lowerrotor assembly 24 b and decreasing the rotor speed of the other of upperrotor assembly 24 a and lower rotor assembly 24 b to create a torqueimbalance that provides yaw authority for aircraft 10. In theillustrated embodiment with upper rotor assembly 24 a rotatingcounterclockwise and lower rotor assembly 24 b rotating clockwise whenaircraft 10 is viewed from above, increasing the rotor speed of upperrotor assembly 24 a and decreasing the rotor speed of lower rotorassembly 24 b will cause aircraft 10 to rotate about vertical axis 10 cin the clockwise direction. Similarly, decreasing the rotor speed ofupper rotor assembly 24 a and increasing the rotor speed of lower rotorassembly 24 b will cause aircraft 10 to rotate about vertical axis 10 cin the counterclockwise direction.

As another example, to maintain a stable hover, differential collectivepitch may be used wherein the effective collective pitch of upper rotorassembly 24 a and lower rotor assembly 24 b is held constant whileincreasing the collective pitch of one of upper rotor assembly 24 a andlower rotor assembly 24 b and decreasing the collective pitch of theother of upper rotor assembly 24 a and lower rotor assembly 24 b tocreate a torque imbalance that provides yaw authority for aircraft 10.In the illustrated embodiment with upper rotor assembly 24 a rotatingcounterclockwise and lower rotor assembly 24 b rotating clockwise whenaircraft 10 is viewed from above, increasing the collective pitch ofupper rotor assembly 24 a and decreasing the collective pitch of lowerrotor assembly 24 b will cause aircraft 10 to rotate about vertical axis10 c in the clockwise direction. Similarly, decreasing the collectivepitch of upper rotor assembly 24 a and increasing the collective pitchof lower rotor assembly 24 b will cause aircraft 10 to rotate aboutvertical axis 10 c in the counterclockwise direction.

In a further example, yaw vanes 34 a, 34 b may be operateddifferentially to create yaw moments in response to propulsion downwashgenerated by propulsion system 22 over yaw vanes 34 a, 34 b. As bestseen in FIG. 4C, when yaw vane 34 a is shifted to the left and yaw vane34 b is shifted to the right, when aircraft 10 is viewed from a forwardposition during hover, the propulsion downwash acting on yaw vanes 34 a,34 b creates yaw moments about the center of gravity of aircraft 10 thaturge aircraft 10 to rotate about vertical axis 10 c in thecounterclockwise direction, as indicated by arrow 90. Similarly, as bestseen in FIG. 4D, when yaw vane 34 a is shifted to the right and yaw vane34 b is shifted to the left, when aircraft 10 is viewed from a forwardposition during hover, the propulsion downwash acting on yaw vanes 34 a,34 b creates yaw moments about the center of gravity of aircraft 10 thaturge aircraft 10 to rotate about vertical axis 10 c in the clockwisedirection, as indicated by arrow 92. In the illustrated embodiment, thedifferential positioning of yaw vanes 34 a, 34 b is achieved byoperation of actuators positioned within aft door 18 a of fuselage 18.

As best seen in FIG. 7, yaw vanes 34 a, 34 b are symmetrically disposedand pivotably coupled to the aft end of fuselage 18 and morespecifically to the aft end of aft door 18 a. It is noted that in theillustrated embodiment, a portion of aft door 18 a has been cut away toreveal yaw vane actuators 94 a, 94 b that are mounted within aft door 18a. Yaw vane actuator 94 a is configured to tilt yaw vane 34 a relativeto aft door 18 a about a pivot axis 96 via linkage 98 a that extendsthrough a slot 18 c in aft door 18 a. Yaw vane actuator 94 b isconfigured to tilt yaw vane 34 b relative to aft door 18 a about pivotaxis 96 via linkage 98 b that extends through a slot 18 d in aft door 18a. Yaw vane actuators 94 a, 94 b are independently operated responsiveto commands from flight control system 20 such that yaw vanes 34 a, 34 bmay be collectively or differentially pivoted relative to aft door 18 a.

In addition to using the yaw authority mechanisms described herein toindividually provide yaw authority for aircraft 10 during hover, flightcontrol system 20 can command multiple yaw authority mechanisms tooperate together to provide yaw authority for aircraft 10 during hover.For example, to cause aircraft 10 to rotate about vertical axis 10 c inthe clockwise direction, flight control system 20 could increase therotor speed and increase the collective pitch of upper rotor assembly 24a while decreasing the rotor speed and decreasing the collective pitchof lower rotor assembly 24 b. To cause aircraft 10 to rotate aboutvertical axis 10 c in the counterclockwise direction, flight controlsystem 20 could decrease the rotor speed and decrease the collectivepitch of upper rotor assembly 24 a while increasing the rotor speed andincreasing the collective pitch of lower rotor assembly 24 b. As anotherexample, to cause aircraft 10 to rotate about vertical axis 10 c in theclockwise direction, flight control system 20 could increase the rotorspeed of upper rotor assembly 24 a, decrease the rotor speed of lowerrotor assembly 24 b, shift yaw vane 34 a to the left and shift yaw vane34 b to the right. To cause aircraft 10 to rotate about vertical axis 10c in the counterclockwise direction, flight control system 20 coulddecrease the rotor speed of upper rotor assembly 24 a, increase therotor speed of lower rotor assembly 24 b, shift yaw vane 34 a to theright and shift yaw vane 34 b to the left.

In a further example, to cause aircraft 10 to rotate about vertical axis10 c in the clockwise direction, flight control system 20 could increasethe collective pitch of upper rotor assembly 24 a, decrease thecollective pitch of lower rotor assembly 24 b, shift yaw vane 34 a tothe left and shift yaw vane 34 b to the right. To cause aircraft 10 torotate about vertical axis 10 c in the counterclockwise direction,flight control system 20 could decrease the collective pitch of upperrotor assembly 24 a, increase the collective pitch of lower rotorassembly 24 b, shift yaw vane 34 a to the right and shift yaw vane 34 bto the left. Additionally, to cause aircraft 10 to rotate about verticalaxis 10 c in the clockwise direction, flight control system 20 couldincrease the collective pitch and rotor speed of upper rotor assembly 24a, decrease the collective pitch and rotor speed of lower rotor assembly24 b, shift yaw vane 34 a to the left and shift yaw vane 34 b to theright. To cause aircraft 10 to rotate about vertical axis 10 c in thecounterclockwise direction, flight control system 20 could decrease thecollective pitch and rotor speed of upper rotor assembly 24 a, increasethe collective pitch and rotor speed of lower rotor assembly 24 b, shiftyaw vane 34 a to the right and shift yaw vane 34 b to the left. Usingmore than one and/or different combinations of yaw authority mechanismscan be beneficial depending upon aircraft parameters, flight dynamicsand/or environmental factors including altitude, attitude, temperature,thrust to weight ratio, wind speed, wind direction, desired yaw rate andother considerations known to those having ordinary skill in the art.

In embodiments wherein the rotor disk of coaxial rotor system 24 has aratio to the wingspan of wings 14, 16 on the order of 1 to 1 or greater,differential operations of elevons 30 may be used to complement otheryaw authority mechanisms in hover or as a standalone yaw authoritymechanism in hover. For example, when elevons 30 a, 30 c are tiltedforward (see FIG. 8B) and elevons 30 b, 30 d are tilted aftward (seeFIG. 8C), propulsion downwash generated by propulsion system 22 overelevons 30 creates yaw moments about the center of gravity of aircraft10 that urge aircraft 10 to rotate about vertical axis 10 c in thecounterclockwise direction, as seen from above in FIG. 1E. Similarly,when elevons 30 b, 30 d are tilted forward (see FIG. 8B) and elevons 30a, 30 c are tilted aftward (see FIG. 8C), propulsion downwash generatedby propulsion system 22 over elevons 30 creates yaw moments about thecenter of gravity of aircraft 10 that urge aircraft 10 to rotate aboutvertical axis 10 c in the clockwise direction, as seen from above inFIG. 1E.

In embodiments wherein propulsion assembly 22 is gimbal mounted tofuselage 18 with a single axis gimbal in which propulsion assembly 22 istiltable only forward and aftward relative to fuselage 18, pitch axisthrust vectoring provides translation authority to aircraft 10 in thefore/aft direction 42 in hover (see FIG. 4A). When propulsion assembly22 is tilted aftward relative to fuselage 18, as indicated by dottedpropulsion assembly 22 c, propulsion assembly 22 generates a thrustvector having a vertical component 44 providing thrust-borne lift foraircraft 10 and an aftward component 46 that urges aircraft 10 totranslate in the aftward direction. When propulsion assembly 22 istilted forward relative to fuselage 18, as indicated by dottedpropulsion assembly 22 d, propulsion assembly 22 generates a thrustvector having vertical component 44 providing thrust-borne lift foraircraft 10 and a forward component 48 that urges aircraft 10 totranslate in the forward direction. In such single axis gimbalembodiments of propulsion assembly 22, translation authority foraircraft 10 in the lateral direction is provided by collective operationof yaw vanes 34 a, 34 b to create lateral forces acting on yaw vanes 34a, 34 b in response to propulsion downwash generated by propulsionsystem 22 over yaw vanes 34 a, 34 b. Collectively shifting yaw vanes 34a, 34 b to the left, urges aircraft 10 to translate to the right (seeFIG. 1B). Likewise, collectively shifting yaw vanes 34 a, 34 b to theright, urges aircraft 10 to translate to the left. Coordinated pitchaxis thrust vectoring and collective yaw vane operation provideomnidirectional translation capability to aircraft 10 in hover.Alternatively or additionally, in such single axis gimbal embodiments ofpropulsion assembly 22, aircraft 10 is operable to translate in anydirection by first, rotating aircraft 10 about yaw axis 10 c to adesired fore/aft or longitudinal orientation then second, pitch axisthrust vectoring to translate in the desired direction.

Continuing with the sequential flight-operating scenario, aircraft 10has completed the vertical ascent to a desired elevation in FIG. 2C andmay now begin the transition from thrust-borne lift to wing-borne lift.As best seen from the progression of FIGS. 2C-2E, aircraft 10 isoperable to pitch down from the VTOL orientation toward the biplaneorientation to enable high speed and/or long range forward flight. Asseen in FIG. 2C, aircraft 10 begins the process by tilting propulsionassembly 22 forward relative to fuselage 18 during the climb. In thisconfiguration, propulsion assembly 22 generates a thrust vector having aforward component 100 that not only urges aircraft 10 to travel in theforward direction but also urges aircraft 10 to rotate about pitch axis10 b. As the forward airspeed of aircraft 10 increases, collectiveoperation of elevons 30 can be used to enhance the pitch down rotationof aircraft 10. As seen in FIG. 2D, longitudinal axis 10 a extends outof the horizontal plane H such that aircraft 10 has an inclined flightattitude of about forty-five degrees pitch down. As illustrated, elevons30 are tilted aftward relative to tail assemblies 26 (see FIG. 8C) andplay a progressively larger role in the pitch control of aircraft 10 asthe forward speed and inclined flight attitude increase to anaerodynamic flight condition. At the same time, the tilt of propulsionassembly 22 relative to fuselage 18 is preferably being reduced.

As best seen in FIG. 2E, aircraft 10 has completed the transition to thebiplane orientation with forward rotor assembly 24 a and aft rotorassembly 24 b counter-rotating in generally parallel vertical planes. Inthe biplane orientation, wing 14 is above fuselage 18 and wing 16 isbelow fuselage 18. By convention, longitudinal axis 10 a has been resetto be in the horizontal plane H, which also includes lateral axis 10 b,such that aircraft 10 has a level flight attitude in the biplaneorientation. As forward flight with wing-borne lift requiressignificantly less power than VTOL flight with thrust-borne lift, theoperating speed and/or collective pitch of forward rotor assembly 24 aand aft rotor assembly 24 b may be reduced. In the biplane orientation,the independent control provided by flight control system 20 overelevons 30 and yaw vanes 34 a, 34 b provides pitch, roll and yawauthority for aircraft 10 which may be enhanced or complemented withthrust vectoring of propulsion assembly 22.

For example, collective operations of elevons 30 provide pitch authorityfor aircraft 10 to control, maintain or change the angle of attack ofwings 14, 16 during forward flight. As best seen in FIG. 5A, when eachof elevons 30 is tilted forward (see FIG. 8B), the airflow acrosselevons 30 creates pitch moments having a downward component on elevons30, as indicted by arrows 102 a, 102 b, 102 c, 102 d. Pitch moments 102a, 102 b, 102 c, 102 d urge aircraft 10 to rotate about pitch axis 10 b,increasing the angle of attack of wings 14, 16 and tending to causeaircraft 10 to climb. Similarly, as best seen in FIG. 5B, when each ofelevons 30 is tilted aftward (see FIG. 8C), the airflow across elevons30 creates pitch moments having an upward component on elevons 30, asindicted by arrows 104 a, 104 b, 104 c, 104 d. Pitch moments 104 a, 104b, 104 c, 104 d urge aircraft 10 to rotate about pitch axis 10 b,decreasing the angle of attack of wings 14, 16 and tending to causeaircraft 10 to descend.

As another example, differential operations of elevons 30 provide rollauthority for aircraft 10 to control, maintain or change the roll angleof aircraft 10 during forward flight. As best seen in FIG. 5C, whenelevons 30 b, 30 d are tilted forward (see FIG. 8B) and elevons 30 a, 30c are tilted aftward (see FIG. 8C), the airflow across elevons 30creates roll moments acting generally perpendicularly to elevons 30, asindicted by arrows 112 a, 112 b, 112 c, 112 d. Roll moments 112 a, 112b, 112 c, 112 d urge aircraft 10 to rotate about roll axis 10 a in theroll left direction, as indicated by arrow 112 e. Similarly, best seenin FIG. 5D, when elevons 30 a, 30 c are tilted forward (see FIG. 8B) andelevons 30 b, 30 d are tilted aftward (see FIG. 8C), the airflow acrosselevons 30 creates roll moments acting generally perpendicularly toelevons 30, as indicted by arrows 114 a, 114 b, 114 c, 114 d. Rollmoments 114 a, 114 b, 114 c, 114 d urge aircraft 10 to rotate about rollaxis 10 a in the roll right direction, as indicated by arrow 114 e.

The operation of elevons 30 is best seen in FIGS. 8A-8C in which ageneric tail assembly 26 is depicted. Tail assembly 26 includes a fixedvertical stabilizer 28 and a tiltable elevon 30 proximate the distal endof tail assembly 26. In the illustrated embodiment, an elevon actuator106 is configured to tilt elevon 30 relative to tail assembly 26 aboutelevon axis 108 via linkage 110 responsive to commands from flightcontrol system 20. When elevon actuator 106 shifts linkage 110 forward,elevon 30 is tilted forward relative to tail assembly 26, as best seenin FIG. 8B. Collectively tilting each elevon 30 of aircraft 10 forwardin this manner creates pitch moments 102 a, 102 b, 102 c, 102 ddescribed above with reference to FIG. 5A. Likewise, when elevonactuator 106 shifts linkage 110 aftward, elevon 30 is tilted aftwardrelative to tail assembly 26, as best seen in FIG. 8C. Collectivelytilting each elevon 30 of aircraft 10 aftward in this manner createspitch moments 104 a, 104 b, 104 c, 104 d described above with referenceto FIG. 5B. As described herein, elevons 30 may be differentiallyoperated wherein some of elevons 30 are tilted forward and some ofelevons 30 are tilted aftward creating, for example, roll moments 112 a,112 b, 112 c, 112 d described above with reference to FIG. 5C or rollmoments 114 a, 114 b, 114 c, 114 d described above with reference toFIG. 5D.

It is noted that the use of the distributed array of elevons 30 operatedby flight control system 20 provides unique advantages for the safetyand reliability of aircraft 10 during flight. For example, in the eventthat flight control system 20 detects a fault with one of the elevons30, flight control system 20 is operable to perform corrective actionresponsive to the detected fault at a distributed elevon level or at acoordinated distributed elevon and propulsion assembly level. As anexample and referring again to FIGS. 5A-5D, flight control system 20 hasdetects a fault in elevon 30 b based upon information received from oneor more sensors or based upon aircraft response to prior actuationcommands. As a first step, flight control system 20 shuts down furtheroperation of elevon 30 b, preferably in a neutral position asrepresented in FIG. 8A. Flight control system 20 now determines whatother corrective measures should be implemented based upon the desiredmaneuvers to be performed by aircraft 10. For example, flight controlsystem 20 may determine that certain operational changes areappropriate, such as selective use or nonuse of the laterally opposedelevon 30 a on upper wing 14, the longitudinally opposed elevon 30 d onlower wing 16 and/or the diametrically opposed elevon 30 c on lower wing16. In addition to corrective action at the distributed elevon level,flight control system 20 can augment such operations by performingcorrective actions with propulsion assembly 22.

For example, to achieve the pitch up maneuver depicted in FIG. 5A duringan elevon 30 b fault, flight control system 20 is configured to leavethe laterally opposed elevon 30 a in the neutral position of FIG. 8A andto actuate elevons 30 c, 30 d to the forward tilt configuration of FIG.8B creating pitch control moments 102 c, 102 d that urge aircraft 10 torotate in the pitch up direction about pitch axis 10 b. In addition,flight control system 20 is configured to coordinate this distributedelevon operation with the upward tilting of propulsion assembly 22 togenerate a thrust vector having an upward component, which also tends tourge aircraft 10 to rotate in the pitch up direction about pitch axis 10b. Similarly, to achieve the pitch down maneuver depicted in FIG. 5Bduring an elevon 30 b fault, flight control system 20 is configured toleave the laterally opposed elevon 30 a in the neutral position of FIG.8A and to actuate elevons 30 c, 30 d in the aftward tilt configurationof FIG. 8C creating pitch control moments 104 c, 104 d that urgeaircraft 10 to rotate in the pitch down direction about pitch axis 10 b.In addition, flight control system 20 is configured to coordinate thisdistributed elevon operation with the downward tilting of propulsionassembly 22 to generate a thrust vector having a downward component,which also tends to urge aircraft 10 to rotate in the pitch downdirection about pitch axis 10 b.

As another example, to achieve the roll left maneuver depicted in FIG.5C during an elevon 30 b fault, flight control system 20 is configuredto leave the diametrically opposed elevon 30 c in the neutral positionof FIG. 8A, to actuate elevon 30 a to the aftward tilt configuration ofFIG. 8C and to actuate elevon 30 d to the forward tilt configuration ofFIG. 8B creating roll control moments 112 a, 112 d that urge aircraft 10to rotate in the roll left direction about roll axis 10 a. In addition,flight control system 20 is configured to coordinate this distributedelevon operation with the creation of a torque imbalance with propulsionassembly 22. This is achieved by increasing the collective pitch offorward rotor assembly 24 a, decreasing the collective pitch of aftrotor assembly 24 b or both and/or by increasing the rotor speed offorward rotor assembly 24 a, decreasing the rotor speed of aft rotorassembly 24 b or both, which also tends to urge aircraft 10 to rotate inthe roll left direction about roll axis 10 a. Similarly, to achieve theroll right maneuver depicted in FIG. 5D during an elevon 30 b fault,flight control system 20 is configured to leave the diametricallyopposed elevon 30 c in the neutral position of FIG. 8A, to actuateelevon 30 a to the forward tilt configuration of FIG. 8B and to actuateelevon 30 d to the aftward tilt configuration of FIG. 8C creating rollcontrol moments 114 a, 114 d that urge aircraft 10 to rotate in the rollright direction about roll axis 10 a. In addition, flight control system20 is configured to coordinate this distributed elevon operation withthe creation of a torque imbalance with propulsion assembly 22. This isachieved by decreasing the collective pitch of forward rotor assembly 24a, increasing the collective pitch of aft rotor assembly 24 b or bothand/or by decreasing the rotor speed of forward rotor assembly 24 a,increasing the rotor speed of aft rotor assembly 24 b or both, whichalso tends to urge aircraft 10 to rotate in the roll right directionabout roll axis 10 a.

It is noted that if elevon 30 b is not shut down in the neutral positionas represented in FIG. 8A but instead becomes frozen in an activeposition such as that of FIG. 8B or 8C, flight control system 20 isconfigured to take corrective action to overcome this elevon fault atthe distributed elevon level. For example, as best seen in FIG. 5E, ifelevon 30 b becomes frozen in the tilt forward position depicted in FIG.8B generating pitch and roll moment 116 b, flight control system 20 isconfigured to actuate the longitudinally opposed elevon 30 d to the tiltaftward position depicted in FIG. 8C to generate an opposing pitch androll moment 116 d. Similarly, as best seen in FIG. 5F, if elevon 30 bbecomes frozen in the tilt aftward position depicted in FIG. 8Cgenerating pitch and roll moment 118 b, flight control system 20 isconfigured to actuate the longitudinally opposed elevon 30 d to the tiltforward position depicted in FIG. 8B to generate an opposing pitch androll moment 118 d. In either of these scenarios, flight control system20 is configured to coordinate this distributed elevon operation withthe upward or downward tilting of propulsion assembly 22 to generate athrust vector having the desired component, to urge aircraft 10 torotate in the desired direction about pitch axis 10 b for pitchauthority. Likewise, flight control system 20 is configured tocoordinate this distributed elevon operation with the creation of atorque imbalance with propulsion assembly 22 by changing the collectivepitch and/or rotor speed of forward rotor assembly 24 a and/or aft rotorassembly 24 b as required to urge aircraft 10 to rotate in the desiredroll direction about roll axis 10 a.

As discussed herein, the distributed array of elevons 30 operated byflight control system 20 provides numerous and redundant paths tomaintain the airworthiness of aircraft 10, even when a fault occurswithin the distributed array of elevons 30. In addition to takingcorrective action at the distributed elevon level or at the coordinateddistributed elevon and propulsion assembly level responsive to adetected fault, flight control system 20 is also operable to change theflight plan of aircraft 10 responsive to the detected fault. Forexample, based upon factors including the extent of the fault or faults,weather conditions, the type and criticality of the mission, thedistance from mission goals and the like, flight control system 20 maycommand aircraft 10 to travel to a predetermined location, to perform anemergency landing or to continue the current mission.

Continuing with the sequential flight-operating scenario, aircraft 10remains in the biplane orientation with forward rotor assembly 24 a andaft rotor assembly 24 b generating forward thrust and with upper wing 14and lower wing 16 generating wing-borne lift in FIG. 2E. In the biplaneorientation, flight control system 20 independently controls yaw vanes34 a, 34 b to provide yaw authority for aircraft 10. For example, yawvanes 34 a, 34 b may be operated collectively to create yaw momentsresponse to the airflow around fuselage 18 and across yaw vanes 34 a, 34b. As best seen in FIG. 5G, when yaw vanes 34 a, 34 b are shifted to theleft, the airflow across yaw vanes 34 a, 34 b creates yaw moments aboutthe center of gravity of aircraft 10 that urge aircraft 10 to rotateabout vertical axis 10 c in a yaw left or counterclockwise direction,when viewed from above, as indicated by arrow 120. Similarly, as bestseen in FIG. 5H, when yaw vanes 34 a, 34 b are shifted to the right, theairflow across yaw vanes 34 a, 34 b creates yaw moments about the centerof gravity of aircraft 10 that urge aircraft 10 to rotate about verticalaxis 10 c in a yaw right or clockwise direction, when viewed from above,as indicated by arrow 122. In the illustrated embodiment, collective yawvane positioning is achieved by operation of yaw vane actuators 94 a, 94b (see FIG. 7) to shift yaw vanes 34 a, 34 b in the same directionrelative to fuselage 18 responsive to commands from flight controlsystem 20.

It is noted that the use of propulsion assembly 22 provides uniqueadvantages for maintaining the airworthiness of aircraft 10 in a oneengine inoperable event during forward flight. In the illustratedembodiment, forward rotor assembly 24 a and aft rotor assembly 24 b ofcoaxial rotors system 24 respectively receive torque and rotationalenergy from electric motors 22 g, 22 h (see FIG. 6). If flight controlsystem 20 detects a fault with electric motor 22 h, for example, flightcontrol system 20 is operable to perform corrective action responsive tothe detected fault including feathering the rotor blades of aft rotorassembly 24 b, which is associated with inoperable motor 22 h, andadjusting the collective pitch and/or rotor speed of forward rotorassembly 24 a, which is driven by the operative motor 22 g. As forwardflight with wing-borne lift requires significantly less power than VTOLflight with thrust-borne lift, operation of a single rotor assemblycoaxial rotors system 24 provides suitable thrust for continued forwardflight. Landing aircraft 10 in the one engine inoperable condition isachieved by transitioning aircraft 10 to the VTOL orientation (see FIGS.2E-2G) and then performing an autorotation and flare recovery maneuverin manned missions with onboard pilot flight control. Alternatively, inboth manned and unmanned missions with autonomous flight control,landing aircraft 10 in the one engine inoperable condition is achievedby transitioning aircraft 10 to the VTOL orientation (see FIGS. 2E-2G)and then deploying a parachute to reduce the descent speed to a landingsurface.

Continuing with the sequential flight-operating scenario, as aircraft 10approaches the destination, aircraft 10 may begin the transition fromwing-borne lift to thrust-borne lift. As best seen from the progressionof FIGS. 2E-2G, aircraft 10 is operable to pitch up from the biplaneorientation to the VTOL orientation to enable, for example, a verticallanding operation. As seen in FIG. 2F, longitudinal axis 10 a extendsout of the horizontal plane H such that aircraft 10 has an inclinedflight attitude of about forty-five degrees pitch up. This can beachieved as discussed herein by collective operation of elevons 30, bythrust vectoring of propulsion assembly 22 or a combination thereof. Asillustrated, this causes an increase in the angle of attack of wings 14,16 such that aircraft 10 engages in a climb. In FIG. 2G, aircraft 10 hascompleted the transition from the biplane orientation to the VTOLorientation and, by convention, longitudinal axis 10 a has been reset tobe in the horizontal plane H, which also includes lateral axis 10 b suchthat aircraft 10 has a level flight attitude in the VTOL orientation.Aircraft 10 may now commence a vertical descent to a landing surface, asbest seen in FIG. 2H. As discussed above, during such VTOL operationsincluding hover operations throughout the landing sequence, flightcontrol system 20 may utilize thrust vectoring of propulsion assembly 22to provide translation authority for aircraft 10 and may utilizedifferential rotor speed control, differential collective pitch controland/or differential yaw vane positioning to provide yaw authority foraircraft 10. As best seen in FIG. 2I, aircraft 10 has landed in atailsitter orientation at the desired location.

Referring now to FIGS. 3A-3F in the drawings, another sequentialflight-operating scenario of aircraft 10 is depicted. As best seen inFIG. 3A, aircraft 10 is in a tailsitter position on a surface such asthe ground or the deck of an aircraft carrier. When aircraft 10 is readyfor a mission, flight control system 20 commences operations providingflight commands to the various systems of aircraft 10. Flight controlsystem 20 may be operating responsive to autonomous flight control,remote flight control, onboard pilot flight control or a combinationthereof. As best seen in FIG. 3B, aircraft 10 has performed a verticaltakeoff and is engaged in thrust-borne lift in the VTOL orientation ofaircraft 10. As illustrated, upper rotor assembly 24 a and lower rotorassembly 24 b are counter-rotating in generally parallel horizontalplanes. As longitudinal axis 10 a and lateral axis 10 b are both in ahorizontal plane H that is normal to the local vertical in the earth'sreference frame, aircraft 10 has a level flight attitude.

During hover, flight control system 20 may utilize speed control and/orcollective pitch control of upper rotor assembly 24 a and lower rotorassembly 24 b to cause aircraft 10 to climb, descend or maintain astable hover. Also during hover, flight control system 20 may utilizethrust vectoring of propulsion assembly 22 to provide translationauthority for aircraft 10 and may utilize differential rotor speedcontrol, differential collective pitch control and/or differential yawvane positioning to provide yaw authority for aircraft 10. As best seenin FIG. 3C, aircraft 10 has completed the vertical ascent to a desiredelevation and may now begin the transition from thrust-borne lift towing-borne lift. As best seen from the progression of FIGS. 3C-3F,aircraft 10 is operable to pitch down from the VTOL orientation towardthe biplane orientation to enable high speed and/or long range forwardflight. As seen in FIG. 3C, aircraft 10 begins the process by tiltingpropulsion assembly 22 forward relative to fuselage 18 from a stablehover, instead of a climb as described above with reference to FIG. 2C.In this configuration, propulsion assembly 22 generates a thrust vectorhaving a forward component 130 that initially causes aircraft 10 totranslate in the forward direction. As the forward airspeed of aircraft10 increases, forward thrust vector component 130 together withcollective aftward tilting of elevons 30 (see FIG. 8C) urge aircraft 10to rotate about pitch axis 10 b in the pitch down direction.

As seen in FIG. 3D, longitudinal axis 10 a extends out of the horizontalplane H such that aircraft 10 has an inclined flight attitude of aboutthirty degrees pitch down. As seen in FIG. 3E, longitudinal axis 10 aextends out of the horizontal plane H such that aircraft 10 has aninclined flight attitude of about sixty degrees pitch down. Asillustrated, elevons 30 are tilted aftward relative to tail assemblies26 (see FIG. 8C) and play a progressively larger role in the pitchcontrol of aircraft 10 as the forward speed and inclined flight attitudeincrease. At the same time, the tilt of propulsion assembly 22 relativeto fuselage 18 is preferably being reduced. As best seen in FIG. 3F,aircraft 10 has completed the transition to the biplane orientation withforward rotor assembly 24 a and aft rotor assembly 24 b counter-rotatingin generally parallel vertical planes. In the biplane orientation, wing14 is above fuselage 18 and wing 16 is below fuselage 18. By convention,longitudinal axis 10 a has been reset to be in the horizontal plane H,which also includes lateral axis 10 b, such that aircraft 10 has a levelflight attitude in the biplane orientation. In the biplane orientation,the independent control provided by flight control system 20 overelevons 30 and yaw vanes 34 a, 34 b provides pitch, roll and yawauthority for aircraft 10 which may be enhanced or complemented withthrust vectoring of propulsion assembly 22. Thus, aircraft 10 isoperable to transition from thrust-borne lift to wing-borne lift takingadvantage of the airspeed established in a climb, as discussed withreference to FIGS. 2A-2E and is also operable to transition fromthrust-borne lift to wing-borne lift taking advantage of the uniquethrust vectoring capability of propulsion assembly 22 to generateforward speed, as discussed with reference to FIGS. 3A-3F.

Referring next to FIG. 9A in the drawings, a systems diagram of anaircraft 200 is depicted. Aircraft 200 is representative of aircraft 10discussed herein. Aircraft 200 includes a propulsion assembly 202, agimbal assembly 204, a flight control system 206, four tail assemblies208 a, 208 b, 208 c, 208 d and a yaw assembly 210. Propulsion assembly202 includes a counter-rotating coaxial rotor system 212 formed fromrotor assembly 212 a and rotor assembly 212 b. Rotor assembly 212 a isoperably associated with an electric motor 214 a and one or morecontrollers, actuators and/or sensors that are generally designated aselectronic systems 216 a, which may specifically include an electronicspeed controller, a collective pitch actuator, a health monitoringsensor and the like. Similarly, rotor assembly 212 b is operablyassociated with an electric motor 214 b and one or more controllers,actuators and/or sensors that are generally designated as electronicsystems 216 b. Propulsion assembly 202 is configured for omnidirectionalthrust vectoring. In the illustrated embodiment, propulsion assembly 202including rotor assembly 212 a, rotor assembly 212 b, electric motor 214a and electric motor 214 b are tilted relative to the fuselage ofaircraft 200 by gimbal assembly 204.

Tail assembly 208 a includes an elevon 218 a that is operably associatedwith one or more actuators, sensors and/or batteries that are generallydesignated as electronic systems 220 a. Tail assembly 208 b includes anelevon 218 b that is operably associated with one or more actuators,sensors and/or batteries that are generally designated as electronicsystems 220 b. Tail assembly 208 c includes an elevon 218 c that isoperably associated with one or more actuators, sensors and/or batteriesthat are generally designated as electronic systems 220 c. Tail assembly208 d includes an elevon 218 d that is operably associated with one ormore actuators, sensors and/or batteries that are generally designatedas electronic systems 220 d. Yaw assembly 210 includes yaw vane 222 aand yaw vane 222 b. Yaw vane 222 a is operably associated with one ormore actuators and/or sensors that are generally designated aselectronic systems 224 a. Yaw vane 222 b is operably associated with oneor more actuators and/or sensors that are generally designated aselectronic systems 224 b.

Flight control system 206 is operably associated with propulsionassembly 202, gimbal assembly 204, tail assemblies 208 a, 208 b, 208 c,208 d and yaw assembly 210. In particular, flight control system 206 islinked to electronic systems 216 a, 216 b, 220 a, 220 b, 220 c, 220 d,224 a, 224 b by a fly-by-wire communications network depicted as arrows226. Flight control system 206 receives sensor data from and sendscommands to propulsion assembly 202, gimbal assembly 204, tailassemblies 208 a, 208 b, 208 c, 208 d and yaw assembly 210 as well asother controlled systems to enable flight control system 206 toindependently control each such system of aircraft 200.

Referring additionally to FIG. 9B in the drawings, a block diagramdepicts a control system 230 operable for use with aircraft 200 oraircraft 10 of the present disclosure. In the illustrated embodiment,system 230 includes two primary computer based subsystems; namely, anaircraft system 232 and a remote system 234. In the illustratedimplementation, remote system 234 includes a programming application 236and a remote control application 238. Programming application 236enables a user to provide a flight plan and mission information toaircraft 200 such that flight control system 206 may engage inautonomous control over aircraft 200. For example, programmingapplication 236 may communicate with flight control system 206 over awired or wireless communication channel 240 to provide a flight planincluding, for example, a starting point, a trail of waypoints and anending point such that flight control system 206 may use waypointnavigation during the mission. In addition, programming application 236may provide one or more tasks to flight control system 206 for aircraft200 to accomplish during a mission. Following programming, aircraft 200may operate autonomously responsive to commands generated by flightcontrol system 206.

In the illustrated embodiment, flight control system 206 includes acommand module 242 and a monitoring module 244. It is to be understoodby those having ordinary skill in the art that these and other modulesexecuted by flight control system 206 may be implemented in a variety offorms including hardware, software, firmware, special purpose processorsand combinations thereof. Flight control system 206 receives input froma variety of sources including internal sources such as sensors 246,electronic systems 216, 220, 224, propulsion assembly 202, gimbalassembly 204, tail assemblies 208 and yaw assembly 210 as well asexternal sources such as remote system 234, global positioning systemsatellites or other location positioning systems and the like. Forexample, as discussed herein, flight control system 206 may receive aflight plan for a mission from remote system 234. Thereafter, flightcontrol system 206 may be operable to autonomously control all aspectsof flight of an aircraft of the present disclosure.

For example, during the various operating modes of aircraft 200including vertical takeoff and landing flight mode, hover flight mode,forward flight mode and transitions therebetween, command module 242provides commands to electronic systems 216, 220, 224. These commandsenable independent operation of each of propulsion assembly 202, gimbalassembly 204, tail assemblies 208 and yaw assembly 210. Flight controlsystem 206 also receives feedback from electronic systems 216, 220, 224.This feedback is processed by monitoring module 244 that can supplycorrection data and other information to command module 242 and/orelectronic systems 216, 220, 224. Sensors 246, such as an attitude andheading reference system (AHRS) with solid-state ormicroelectromechanical systems (MEMS) gyroscopes, accelerometers andmagnetometers as well as other sensors including positioning sensors,speed sensors, environmental sensors, fuel sensors, temperature sensors,location sensors and the like also provide information to flight controlsystem 206 to further enhance autonomous control capabilities.

Some or all of the autonomous control capability of flight controlsystem 206 can be augmented or supplanted by remote flight control from,for example, remote system 234. While operating remote controlapplication 238, remote system 234 is configured to display informationrelating to one or more aircraft of the present disclosure on one ormore flight data display devices 248. Display devices 248 may beconfigured in any suitable form, including, for example, liquid crystaldisplays, light emitting diode displays or any suitable type of display.Remote system 234 may also include audio output and input devices suchas a microphone, speakers and/or an audio port allowing an operator tocommunicate with other remote control operators, a base station, anonboard pilot, crew or passengers on aircraft 200. The display device248 may also serve as a remote input device 250 if a touch screendisplay implementation is used, however, other remote input devices,such as a keyboard or joystick, may alternatively be used to allow anoperator to provide control commands to an aircraft being operatedresponsive to remote control.

As discussed herein, aircraft 10 may be a manned or unmanned aircraftand may operate in many roles including military, commercial, scientificand recreational roles, to name a few. For example, as best seen in FIG.10A, aircraft 10 may be a logistics support aircraft configured forcargo transportation. In the illustrated implementation, aircraft 10 isdepicted as carrying a single package 300 within fuselage 18. In otherimplementations, the cargo may be composed of any number of packages orother items that can be carried within fuselage 18. Preferably, thecargo is fixably coupled within fuselage 18 by suitable means to preventrelative movement therebetween, thus protecting the cargo from damageand maintaining a stable center of mass for aircraft 10. In theillustrated implementation, the cargo may be insertable into andremovable from fuselage 18 via aft door 18 c to enable sequential cargopickup, transportation and delivery operations to and from multiplelocations. In one example, aircraft 10 may provide package deliveryoperations from a warehouse to customers. In another example, aircraft10 may transport weapons or other military hardware to personnel in amilitary theater.

Aircraft 10 may have remote release capabilities in association withcargo transportation. For example, this feature allows aircraft 10 todeliver cargo to a desired location following transportation thereofwithout the requirement for landing. For example, upon reaching thedesired location in a package delivery operation and transitioned fromthe biplane orientation to the VTOL orientation, flight control system20 may cause aft door 18 c to open such that the cargo can be releasedfrom aircraft 10. This feature may also be useful for cargo dropoperations to provide food, water, medicine or other critical items toremote regions during humanitarian or disaster relief missions.Alternatively, as best seen in FIG. 10B, the delivery or pickup of cargomay be accomplished using a cargo hook module 302 that may include acargo hoisting device disposed within fuselage 18 that is operable toraise and lower the cargo while aircraft 10 engages in a stable hover orwhile aircraft 10 rests in a tailsitting position on a surface. Asanother alternative, cargo hook module 302 may represent a cargo hook ona fixed length sling assembly that is operable to suspend the cargo adesired distance from the aft end of aircraft 10 during pickup,transportation and drop off of the cargo.

As best seen in FIG. 10C, aircraft 10 may include a turret mountedsensor assembly 304 that operates one or more sensors such as anintegrated sensor suite. For example, sensor assembly 304 may includeone or more of an infrared sensor such as a forward-looking infrared(FLIR) sensor, a night vision sensor or other optical sensor, a lasersensor, a lidar sensor, a sound sensor, a motion sensor, a highresolution camera, a radar, a multispectral sensor or any other type ofsensor. When aircraft 10 is configured with sensor assembly 304,aircraft 10 may perform a variety of missions including aerialphotography, search and rescue missions, inspection of utility lines andpipelines, environment monitoring, border patrol missions, forest firedetection and monitoring, accident investigation and crowd monitoring,to name a few. In addition, aircraft 10 may engage in militaryoperations such as intelligence, surveillance, target acquisition andreconnaissance. Alternatively, as best seen in FIG. 10D, aircraft 10 maybe configured to engage in attack missions. In the illustratedimplementation, aircraft 10 has a weapons array including fourunder-wing mounted air-to-ground missile systems 306 a, 306 b, 306 c,306 d such as Hellfire or JAGM missile systems. In otherimplementations, the weapons array of aircraft 10 could includeair-to-air missile systems, such as AIM-9 Sidewinder missile systems,and/or anti-submarine torpedo systems such as MK50 torpedo systems.

Even though aircraft 10 has been depicted and described herein as havingparticular attributes, it should be understood by those having ordinaryskill in the art that an aircraft could have alternate structure withoutdeparting from the principles of the present disclosure. For example,aircraft 310 depicted in FIG. 11A shares many common features withaircraft 10 as indicated by the common numbering of common parts.Aircraft 310, however, has straight wings, only wing 312 being visible,instead of the swept wings 14, 16 of aircraft 10. Similarly, aircraft320 depicted in FIG. 11B shares many common features with aircraft 10 asindicated by the common numbering of common parts. Aircraft 320,however, has flat wings 322, 324, instead of dihedral wing 14 andanhedral wing 16 of aircraft 10. As another example, aircraft 330depicted in FIG. 11C shares many common features with aircraft 10 asindicated by the common numbering of common parts. Aircraft 330,however, has a propulsion assembly 332 that includes a counter-rotatingcoaxial rotor system 334 formed from rotor assembly 334 a and rotorassembly 334 b, each of which has two rotor blades instead of four rotorblades as in the rotor assemblies of aircraft 10. Similarly, aircraft340 depicted in FIG. 11D shares many common features with aircraft 10 asindicated by the common numbering of common parts. Aircraft 340,however, has a propulsion assembly 342 that includes a counter-rotatingcoaxial rotor system 344 formed from rotor assembly 344 a and rotorassembly 344 b, each of which has five rotor blades instead of fourrotor blades as in the rotor assemblies of aircraft 10.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. An aircraft operable to transition betweenthrust-borne lift in a VTOL orientation and wing-borne lift in a biplaneorientation, the aircraft comprising: an airframe including a firstwing, a second wing and a fuselage that extends between the first andsecond wings; a propulsion assembly coupled to the fuselage, thepropulsion assembly including a counter-rotating coaxial rotor systemthat is tiltable relative to the fuselage to generate a thrust vector;first and second yaw vanes extending aftwardly from the fuselage; and aflight control system configured to direct the thrust vector of thecoaxial rotor system and control movements of the yaw vanes; wherein, inthe VTOL orientation, differential operation of the yaw vanes providesyaw authority for the aircraft; and wherein, in the biplane orientation,collective operation of the yaw vanes provides yaw authority for theaircraft.
 2. The aircraft as recited in claim 1 wherein the propulsionassembly further comprises a motor assembly and wherein the coaxialrotor system and the motor assembly are tiltable relative to thefuselage to generate the thrust vector.
 3. The aircraft as recited inclaim 1 wherein the coaxial rotor system is configured to provide thrustin line with a yaw axis of the aircraft in the VTOL orientation and inline with a roll axis of the aircraft in the biplane orientation.
 4. Theaircraft as recited in claim 1 wherein, in the VTOL orientation, thefirst wing is forward of the fuselage and the second wing is aft of thefuselage; and wherein, in the biplane orientation, the first wing isbelow the fuselage and the second wing is above the fuselage.
 5. Theaircraft as recited in claim 1 wherein the first and second yaw vanesare pivotably coupled to the fuselage.
 6. The aircraft as recited inclaim 1 wherein, in the VTOL orientation, the aircraft has a VTOL yawaxis that extends through the fuselage and the propulsion assembly; andwherein, in the biplane orientation, the aircraft has a biplane yaw axisthat extends through the fuselage and the wings.
 7. The aircraft asrecited in claim 6 wherein, in the VTOL orientation, the first andsecond yaw vanes are symmetrically disposed relative to the VTOL yawaxis.
 8. The aircraft as recited in claim 6 wherein, in the biplaneorientation, the first and second yaw vanes are aftwardly disposedrelative to the biplane yaw axis.
 9. The aircraft as recited in claim 6wherein, in the VTOL orientation, the first yaw vane is actuated in afirst direction and the second yaw vane is actuated in a seconddirection that is opposite of the first direction to provide yawauthority for the aircraft.
 10. The aircraft as recited in claim 9wherein, in the VTOL orientation, downwash generated by the propulsionassembly acting on the actuated first and second yaw vanes creates firstand second yaw moments that urge the aircraft to rotate about the VTOLyaw axis.
 11. The aircraft as recited in claim 6 wherein, in the biplaneorientation, the first and second yaw vanes are both actuated in a firstdirection to provide yaw authority for the aircraft.
 12. The aircraft asrecited in claim 11 wherein, in the biplane orientation, airflowgenerated responsive to the forward airspeed of the aircraft acting onthe actuated first and second yaw vanes creates yaw moments that urgesthe aircraft to rotate about the biplane yaw axis.
 13. An aircraftoperable to transition between thrust-borne lift in a VTOL orientationand wing-borne lift in a biplane orientation, the aircraft comprising:an airframe including a first wing, a second wing and a fuselage thatextends between the first and second wings; a propulsion assemblycoupled to the fuselage, the propulsion assembly including acounter-rotating coaxial rotor system having first and second rotorassemblies, the coaxial rotor system tiltable relative to the fuselageto generate a thrust vector; first and second yaw vanes extendingaftwardly from the fuselage; and a flight control system configured todirect the thrust vector of the coaxial rotor system, control rotorspeeds and collective pitches of the first and second rotor assembliesand control movements of the yaw vanes; wherein, in the VTOLorientation, differential operation of the first and second rotorassemblies and differential operation of the yaw vanes provide yawauthority for the aircraft.
 14. The aircraft as recited in claim 13wherein the first and second yaw vanes are pivotably coupled to thefuselage.
 15. The aircraft as recited in claim 13 wherein, in the VTOLorientation, the aircraft has a VTOL yaw axis that extends through thefuselage and the propulsion assembly; and wherein, in the biplaneorientation, the aircraft has a biplane yaw axis that extends throughthe fuselage and the wings.
 16. The aircraft as recited in claim 15wherein, in the VTOL orientation, the first and second yaw vanes aresymmetrically disposed relative to the VTOL yaw axis.
 17. The aircraftas recited in claim 15 wherein, in the VTOL orientation, when the firstyaw vane is actuated in a first direction and the second yaw vanes isactuated in a second direction that is opposite of the first direction,downwash generated by the propulsion assembly acting on the first andsecond yaw vanes creates first and second yaw moments that urge theaircraft to rotate about the VTOL yaw axis.
 18. The aircraft as recitedin claim 13 wherein, in the VTOL orientation, the flight control systemis configured to provide yaw authority for the aircraft responsive toactuating the first yaw vane in a first direction, actuating the secondyaw vane in a second direction that is opposite of the first directionand differentially operating the first and second rotor assemblies byadjusting the rotor speeds.
 19. The aircraft as recited in claim 13wherein, in the VTOL orientation, the flight control system isconfigured to provide yaw authority for the aircraft responsive toactuating the first yaw vane in a first direction, actuating the secondyaw vane in a second direction that is opposite of the first directionand differentially operating the first and second rotor assemblies byadjusting the collective pitches.
 20. The aircraft as recited in claim13 wherein, in the VTOL orientation, the flight control system isconfigured to provide yaw authority for the aircraft responsive toactuating the first yaw vane in a first direction, actuating the secondyaw vane in a second direction that is opposite of the first directionand differentially operating the first and second rotor assemblies byadjusting the rotor speeds and the collective pitches.